Remove a lot of dead code from AXNodeObject

Source/core/accessibility/AXObject.cpp

Index: Source/core/accessibility/AXObject.cpp
diff --git a/Source/core/accessibility/AXObject.cpp b/Source/core/accessibility/AXObject.cpp
index 4049f914a6f0bc06f315da8ff7658af8a96ab52a..413620b3bf9cb0b041db893fc57af832ae4f6c0e 100644
--- a/Source/core/accessibility/AXObject.cpp
+++ b/Source/core/accessibility/AXObject.cpp
@@ -395,15 +395,6 @@ AccessibilityButtonState AXObject::checkboxOrRadioValue() const
     return ButtonStateOff;
 }
 
-const AtomicString& AXObject::placeholderValue() const
-{
-    const AtomicString& placeholder = getAttribute(placeholderAttr);
-    if (!placeholder.isEmpty())
-        return placeholder;
-
-    return nullAtom;
-}
-
 bool AXObject::ariaIsMultiline() const
 {
     return equalIgnoringCase(getAttribute(aria_multilineAttr), "true");